Fix customization file resolution errors for remote agent host sessions

[roblourens]

When a remote agent host session is active, SessionsAICustomizationWorkspaceService.getActiveProjectRoot() returns the session's repository URI, which is a vscode-agent-host:// URI. The AgenticPromptFilesLocator then uses this as a workspace folder root and tries to resolve .github/prompts and .github/hooks against it, causing errors like:\n\n\nFailed to resolve files at location: vscode-agent-host://localhost__8080/file/-/datadrive2/debugtest/.github/prompts No connection for authority: localhost__8080\n\n\nThe fix filters out AGENT_HOST_SCHEME URIs in both the activeProjectRoot derived observable and the getActiveProjectRoot() getter, so the project root returns undefined for remote sessions — consistent with the existing guard in WorkspaceFolderManagementContribution.\n\n(Written by Copilot)"

[Copilot]

Pull request overview

Fixes AI customization file discovery in the Sessions window when the active session is backed by a remote agent host, by preventing vscode-agent-host:// URIs from being treated as a workspace root (which causes .github/prompts / .github/hooks resolution failures).

Changes:

• Filter out AGENT_HOST_SCHEME (vscode-agent-host) in the activeProjectRoot derived observable.
• Apply the same filtering in the getActiveProjectRoot() getter for consistency with observable consumers.